The present invention relates to a method, whereby the non-ferrous metal content of the slag generated in the production of non-ferrous metals such as copper or nickel in a suspension smelting furnace is reduced by feeding metallurgical coke, whose size ranges from 1-25 mm, into the furnace. It is advantageous to place baffles from the roof of the furnace downwards, by means of which small particles containing copper and nickel are prevented from drifting to the back of the furnace and exiting together with the slag. The baffles force small particles to settle in the reduction zone of the furnace.
It is known before that slag with low copper content can be produced in suspension smelting furnaces such as flash smelting furnaces, when fixed coke or some other carbonaceous substance is used in the reduction of slag and the copper oxidule dissolving therein and especially magnetite which increases the viscosity of the slag and slows down the separation of molten matte particles contained in the slag by settling.
In U.S. Pat. No. 5,662,370 a method is described in which it is essential that the carbon content of the carbonaceous material to be fed to the reaction shaft is at least 80%, that at least 65% of the material particles are under 100 xcexcm and at least 25% between 44-100 xcexcm. Particle size is defined precisely, because, according to said patent, the reduction of magnetite with unburnt coke occurs under two mechanisms and particle size is of decisive significance with regard to said mechanisms. If the rough coke powder size is roughly 100 xcexcm or greater, the unburnt part particle size is also great and for this reason coke remains floating on the slag surface and reactions are slow. When the particle size is reduced, the powder coke enters the slag and then into direct contact with the magnetite to be reduced, which accelerates the reaction rate.
In Japanese patent application 58-221241 a method is described in which coke breeze or coke breeze together with pulverized coal are fed into the reaction shaft of a flash smelting furnace through a concentrate burner. The coke is fed into the furnace so that the entire surface of melt in the lower furnace is evenly covered with the unburnt powder coke. According to the application, the degree of reduction of magnetite decreases when the grain size is ultra-fine, so the grain size used is preferably from 44 xcexcm to 1 mm. The slag layer covered by unburnt coke, which remains on the molten slag bath decreases considerably the partial pressure of oxygen. The highly reducing atmosphere arising from the coke layer causes for example damages to the lining of the furnace.
In JP patent 90-24898 a method is described in which pulverized coke or coal with particle size of under 40 mm is fed into a flash smelting furnace to replace the oil used as an extra fuel and maintain the desired temperature in the furnace.
JP patent application 9-316562 applies to the same method as the previously mentioned U.S. Pat. No. 5,662,370. The difference from the method of the US patent is that carbonaceous material is fed to the lower part of the reaction shaft of the flash smelting furnace, to prevent said carbonaceous material from burning before it reaches the slag and the magnetite to be reduced contained therein. The particle size of the carbonaceous material is essentially the same as the distribution described in the US patent.
In some of the previously described methods the small particle size of the coke presents a weakness, in that small coke particles do not settle at all from the gas phase but continue with the gas phase to the uptake and on to the waste-heat boiler as a reducing agent. In the boiler the coke particles react and generate unnecessary energy in the wrong place, which may even limit total process capacity as the waste-heat boiler capacity diminishes.
In a suspension smelting furnace, not only does pulverized material such as cuprous oxides drift with the gas phase to the back of the furnace and the uptake but also copper matte particles. When these small particles separate from the gas flow in the back of the furnace and settle to the surface of the slag phase, this phenomenon is very slow due precisely to the small particle size. Because slag is mainly tapped from the back or side of the furnace, these particles do not manage to settle through the slag phase but instead, they drift in connection with slag tapping out of the furnace and add to the copper content of the slag.
In order to solve the previously described problems, a method has now been developed, with which the drawbacks of previous methods can be avoided. In the newly developed method, the aim is to lower the non-ferrous metal content of the slag generated in the production of non-ferrous metals such as copper or nickel in a suspension smelting furnace so that the slag would be discardable slag that would not require further processing. In this method, metallurgical coke, whose size ranges from 1-25 mm, is used to reduce the slag wherein most of the coke to be fed through the reaction shaft separates in the lower furnace of the suspension smelting furnace from the gas phase and settles on the surface of the slag phase, in which reduction of the slag occurs in an area where the majority of the product obtained as matte and slag separates from each other. The essential features of the invention will become apparent in the attached patent claims.
In this method, it is preferable to use metallurgical coke, because the amount of volatile substances contained therein is small. Therefore, the major part of the reduction potential of the raw materials in question can be used in reduction, without generating redundant additional thermal energy when the volatile substances in the reducing material burn. At the same time, the number of oxygen-binding reactions which happen to the coke in the reaction shaft is decreased, which allows for better control of the quality of the resulting matte. Traditionally, this control has been achieved by adjusting the air co-efficient in the process (oxygen/concentrate amount Nm3/t).
In the method of the present invention, the metallurgical coke used is of a certain grain size, so that most of the coke to be fed through the reaction shaft separates from the gas phase in the lower furnace of the suspension smelting furnace and settles on the surface of the slag phase where the slag reduction takes place in an area in which also matte and slag which are main part of the products, separate from the gas phase. Reduction takes place in the area optimal from the point of heat economy: the heat required for reduction comes from the heat content of the products coming from the reaction shaft, without any additional energy being required in reduction.
The grain size of the metallurgical coke is preferably 1-25 mm. Bigger size coke has such a small specific area, that it will not react effectively with the slag. If a smaller grain size is used, such as the previously mentioned 1-25 mm, the coke will react actively already in the reaction shaft and more of it will drift with the gas phase to the uptake and the desired slag contact and reduction effect will be poor. When fine grained coke drifts with the gas phase to the uptake and/or waste-heat boiler, it produces energy at a stage when it is not needed and will thereby reduce the capacity of the boiler. The coke feed is controlled in such a way that a considerable amount of coke does not build up in the furnace, at most only a few centimetres but instead, all the coke is consumed in the reduction reactions.
In the method of the present invention also, the settling of pulverized matte material on the surface of the slag phase still causes the same problem to some extent as previously described: small particles containing copper or nickel do not manage to settle through the slag phase but stay in the slag, thereby raising the copper and nickel content of the slag being tapped off. In our method, this problem is preferably overcome in the way described: by positioning baffles from the roof of lower furnace section of the suspension smelting furnace. These will hinder the drifting of fine grained particles with the gas phase to the back of the furnace near the tapping holes. The baffles are positioned from the furnace roof downwards so that at their lower part, they reach either the molten slag bath or near its surface. The baffles are preferably constructed from water-cooled copper elements, which are protected with a fireproof material such as brick or refractory masses.
Thanks to the baffles, matter containing the most fine grained copper or nickel is made to settle in the reduction zone. In this way, the slag in the tapping area no longer contains substances forming of non-ferrous metal particles that settle slowly and increase copper content of the slag. The slag that is tapped from the tapping hole has a lower copper or nickel content than when operating without coke reduction and baffles.